Production of foamable vinyl aromatic resin compositions



, No Drawing.

United States Patent 3,026,274 PRGDUCTIQN 6F FOAMABLE ViNYL AROMATICRESEJ COWGSITEONS William J. M n /lillan, Midland, Mich, and George L.

Nieholls, Dubuque, Iowa, assignors to The Dow Chernical Company,Midland, Mich a corporation of Delaware Continuation of application Ser.No. 699,526, Nov. 29, 1957. This application May 8, 1961, Ser. No.110,605

6 Claims. (Cl. 260-25) This invention relates to the production offoamable vinyl aromatic resin compositions. It relates more particularlyto a process for making latent-foaming polymeric compositions comprisedessentially of a normally solid thermoplastic vinyl aromatic resinhaving a volatile organic compound uniformly dispersed throughout andpertains especially to a method of making such polymeric compositionshaving improved foaming characteristics.

A process for making latent-foaming polymeric compositions is describedin U.S. application Serial No. 699,666, filed November 29, 1957, by EarlF. Engles, now abandoned. In brief the process consists in forming ahomogeneously heat-plastified mass comprising essentially a uniformmixture of a normally solid thermoplastic vinyl aromatic resin, e.g.polystyrene, polyvinyltoluene, polydichlorostyrene or the like, and avolatile organic compound in which the polymer is substantiallyinsoluble such as a saturated aliphatic hydrocarbon containing from 4 to7 carbon atoms in the molecule or sym-dichlorotetrafluoroethane, undersuperatmospheric pressure and extruding the heat-plastified mass throughan orifice suitably in the form of strands, ribbons or a thin sheet at atemperature between 90 and 130 C. into a zone of substantiallyatmospheric pressure and almost immediately contacting the extrudedmaterial with a cooling liquid heat-transfer medium such as water orbrine and rapidly cooling the extruded material to a temperature below50 C., preferably to a temperature between and 30 C. whereby a normallysolid polymeric composition having a density of from about 0.8 to 1.2and consisting essentially of the solid thermoplastic vinyl aromaticresin having the volatile organic compound uniformly dispersedthroughout, is obtained. Such polymeric compositions upon heating to thesoftening point of the polymer, or a somewhat higher temperature, andabove the boiling point of the volatile organic compound containedtherein as foaming agent, expand to form a cellular or porous mass.

In making cellular masses from latent-foaming polymeric compositionsprepared by the procedure described in the aforementioned patentapplication it has been observed that the foaming characteristics of thecompositions vary widely. More specifically, t-he foamed volume of thepolymer may vary from as little as three up to thirty times the initialvolume of the polymeric composition between compositions prepared fromsimilar vinyl aromatic resins containing substantially the sameproportion and kind of volatile organic compound as the foaming agent.In addition to the wide variation in the foam volume which has asubstantial efiect on the density of the foam that is obtained, it hasalso been observed that the size and the uniformity of the cells in theexpanded polymer-is subject to wide variation, in that a single piece ofthe foamed polymer is frequently found to consist of both fine andrelatively large cells of nonice uniform sizes. The cause of theobserved variations in the foaming characteristics of the latent-foamingvinyl aromatic resin compositions and the non-uniformity in the sizes ofthe cells in the foamed masses prepared from compositions which containsubstantially the same kind and proportions of ingredients is not known,but is thought to be caused in part by the inherent hot-stretching ororienting of the extruded material as it falls, or is drawn away fromthe face of the extrusion die and cooled. The cooling of the material totemperatures below the softening point of the polymer not only locks-inthe volatile organic compound employed as the foaming agent, but alsofreezes the polymer in an oriented or strained condition. Upon heatingof the latent foaming polymeric compositions it has been observed thatthe polymer first shrinks slightly, indicating relaxation of strains,then expands to a cellular mass. Regardless of the cause, it is evidentthat such variation in the foaming characteristics of the vinyl aromaticresin compositions renders them less suited for the intended use than isdesired.

It has now been discovered that substantial uniformity in the foamingcharacteristics, both with regard to the volume of the foamed mass andin the formation of cellular bodies composed of uniform small cells ofsubstantially the same size, can readily be obtained by subjecting thelatent-foaming vinyl aromatic resin compositions, having the solidpolymer in an oriented or strained condition, to a heat-treatment incontact with a liquid heattransfer medium at temperatures below thesoftening point of the polymer whereby the oriented polymer issubstantially relaxed without substantial swelling or expanding of thepolymer and cooling of the relaxed polymer to its normally solidcondition, after which it can be foamed or stored and subsequentlyfoamed to form a cellular mass of low density and composed of uniformsmall cells.

The process can advantageously be employed for treating latent-foamingvinyl aromatic resin compositions prepared by procedure as described inthe afore-mentioned patent application, i.e. wherein a normally solidthermoplastic vinyl aromatic resin such as polystyrene or the like and avolatile organic compound, e.g. pentane or dichlorotetrafiuoroethane,which is a poor solvent for the polymer is malaxated undersuperatmospheric pressures and at elevated temperatures into ahomogeneously heatplastified mass, then is extruded into a zone ofsubstantially atmospheric pressure and almost immediately cooled to atemperature below C. In most instances the soprepared latent-foamingvinyl aromatic compositions contain the polymer in an oriented orstrained condition, and have been found to have their foamingcharacteristics substantially improved when subjected to aheattreatment, which can include a partial foaming or prefoaming of thecomposition, according to the invention.

The process can be employed to improve the foaming characteristics oflatent-foaming polymeric compositions containing the polymer in orientedor strained condition, which compositions are comprised essentially of athermoplastic vinyl aromatic resin having a volatile organic compound inwhich the polymer is substantially insoluble uniformly dispersedthroughout.

The vinyl aromatic resin can be a normally solid thermoplastic polymerof one or more monovinyl aromatic hydrocarbons of the benzene series,i.e. having a single vinyl radical directly attached to a carbon atom ofthe benzene nucleus, or a nuclear halogenated derivative thereof.Examples of suitable monomers from which the vinyl aromatic resins canbe prepared are styrene, vinyltoluene, vinylxylene, ethylvinylbenzene,isopropylstyrene, ar-chlorostyrene, ar-dichlorostyrenear-chlorovinyltoluene, fluorostyrene, bromostyrene or tert.-bu-tylstyrene. Copolymers of from 70 to 99 percent by weight of any one ormore of such monovinyl aromatic compounds and from 30 to 1 percent ofalpha-methyl styrene, alphaethyl styrene,para-methyl-alpha-methylstyrene or acrylonitrile can also be used.

The volatile organic compounds which can be employed as the foamingagent for the vinyl aromatic resin can be a saturated aliphatichydrocarbon containing from 4 to 7 carbon atoms in the molecule or aperchlorofluorohydrocarbon, which volatile organic compound has amolecular weight of at least 58 and a boiling point not higher than 95C. at atmospheric pressure. Examples of such volatile organic compoundsare n-pentane, isopentane, neopentane, hexane, n-butane, isobutane,heptane, octane, dichlorodifluoromethane, trichlorofluoromethane,monochlorotrifluoromethane, sym-tetrachlorodifluoroethane,1,2,2-trichloro,1,1,2-trifluoroethane, symdichlorotetrafiuoroethane,l-chloro-l,l,2,2,2-pentafluoroethane,l-chloro-1,2,2,3,3,4,4-heptafluorocyclobutane, or1,Z-dichloro-l,2,3,3,4,4-hexafiuorocyclobutane. Mixtures of any two ormore of such volatile organic compounds can also be employed as thefoaming agent. Such agent or mixture of agents are usually employed inamounts corresponding to from 0.05 to 0.3 gram molecular proportion ofthe volatile organic compound(s) per 100 grams of the thermoplasticvinyl aromatic resin starting material.

In practice of the invention the latent-foaming vinyl aromatic resincompositions having the normally solid thermoplastic polymer in anoriented or strained condition and containing the volatile organiccompound uniformly dispersed throughout is contacted with a liquid bathof a heat-transfer medium, preferably water or brine, at temperaturesbetween 50 and 75 C. for a time sufficient to cause substantialrelaxation of the strains in the polymer and insufficient to produceappreciable expanding or foaming of the polymer. In general, a treatingtime of from 2 to 180 minutes in water at temperatures between 50 and 75C. is satisfactory. The time of treating the latent-foaming polymericcomposition will vary, depending in part upon the size orcross-sectional dimensions of the polymerized composition and in partupon the temperature of the heat-transfer medium. For small particles,e.g. beads, rods, strands, etc., of sizes having a cross-section ordiameter between & and A inch a relative short time of heat-treatingsuch as from 2 to minutes in a water bath at temperatures between 50 and75 C. is satisfactory, whereas large particles, beads or strands requirea somewhat longer time within the limits herein stated to obtain thebeneficial results of the'invention. The latent-foaming polymeric com;positions should not be heated at temperatures and for prolonged periodsof time which cause substantial foaming of the polymer or appreciableloss of the volatile organic foaming agent from the polymer, although apartial foaming or pre-foaming of the latent-foaming compositions canadvantageously be employed or carried out. Such pre-foamed beads orparticles can advantageously be employed in the subsequent production offoamed articles wherein the pre-foamed beads are further processed andexpanded in a mold and caused to flow together during the expanding toobtain a cellular article having a desired shape or are expanded in aclosed container to fill or to substantially'fill the container with acellular mass of the polymer, e.g. refrigerator doors.

As previously stated the heat-transfer medium is preferably liquid Wateror brine, but other aqueous inert liquid media can be used as mixturesof Water and glycerine, ethylene glycol, ethyl alcohol, methyl alcoholor isopropyl alcohol, which mixtures contain a major proportion byweight of water.

The process can be carried out batchwise or in continuous manner. Forexample, the latent-foaming polymeric composition in the form of smallparticles can be immersed in a water bath and heat-treated attemperatures between 50 C. and 75 C. for a time of from 2 to 180minutes, then removed and cooled to a lower temperature and dried. In analternate procedure, the latentfoaming polymeric composition can beprepared by extruding a uniform homogeneously heat-plastified mass ofthe thermoplastic vinyl aromatic resin and the volatile organic compoundin the desired proportions and under superatmospheric pressure at atemperature between and 130 C. through an extrusion orifice as one or aplurality of strands into a zone of substantially atmospheric pressurewherein it is allowed to fall away or is drawn away from the extrusionorifice and almost immediately cooled to a temperature below 50 C. Thecooled material in the form of strands having the polymer in oriented orstrained condition is then passed in continuous manner, as it is formedand cooled, into or through a liquid heat-transfer medium and is heatedat temperatures between 50 and 75 C., whereby the oriented polymer issubstantially relaxed, then is cooled to a lower temperature such as atemperature between 10 and 30 C. by contact with water or brine or otherliquid heat-transfer medium before appreciable expansion of the polymeror substantial loss of the volatile foaming agent occurs. Thereafter,the heat-treated and cooled latent-foaming polymeric material is usuallycut or ground to a granular form. The heat-treated latentfoamingpolymeric compositions possess substantially uniform foamingcharacteristics and are useful for making foamed masses having a lowdensity and composed of cells of uniform size.

The following examples illustrate Ways in which the principle of theinvention has been applied but are not to be construed as limiting itsscope.

EXAMPLE 1 A latent-foaming polymeric composition consisting of solidpolystyrene containing 5.9 percent by weight of isopeutane uniformlydispersed throughout was prepared by supplying heat-plastifiedpolystyrene having a viscosity characteristic of 25 centipoises asdetermined for a 10 weight percent solution of the polystyrene intoluene at 25 C., to a plastics extruder at a temperature of about 180C. wherein it was mixed with 6 percent by weight of isopentane andmalaxated under a superatmospheric pressure and cooled to a uniformhomogeneously heatplastified mass and extruded under a pressure of about850 pounds per square inch gauge pressure at a temperature of about 112C. through a A -inch diameter drill hole into a zone of substantiallyatmospheric pressure and almost immediately passed downward into a bodyof water maintained at temperatures between 10 and 23 C. The cooledmaterial was drawn away as a strand having a diameter of about /s-inchand was cut to a granular form in lengths of about Ai-inch. The productcontained 5.9 percent by weight of isopentane. The polymer was instrained or oriented conditions as determined by viewing the same underpolarized light. Test portions of the polystyrene composition wereimmersed in water at a temperature hereinafter stated and heated at saidtemperature for a time as stated in the following table, then removedand cooled to room temperature. Thereafter, the sotreated polymericcomposition was foamed by heating the same at temperatures between andC. with superheated steam at atmospheric pressure for a period of 3minutes. The foamed product was examined for total volume compared tothe initial volume of the treated beads and the size of the cells.

Table I identifies the experiments and gives the ratio of the foamedmass to the initial volume of the polymeric material and the size of theindividual cells. For purpose of comparison a portion of the polymericmaterial which Table I Heat Treatment Foamed Product Run N0. Foam Sizeof Water Time, Volume] Cells, Temp., Min. initial 0. volume ratioEXAMPLE 2 A latent-foaming polymeric composition consisting ofpolystyrene containing 5 percent by weight of neopentane and in the formof /8 X fls-inch pellets, prepared by procedure similar to thatdescribed in Example 1, was heattreated by immersing the same in waterat a temperature turd for a time as stated in the following table, thenremoving the treated material and cooling the same to room temperature.Portions of the treated polymeric material were foamed employingprocedure similar to that employed in Example 1. The results arereported in Table II.

A granular latent-foaming polymeric composition consisting ofpolystyrene containing one percent by weight of white mineral oil aslubricant and 6.29 percent by Weight of isopentane uniformly dispersedthroughout as foaming agent was prepared by procedure similar to thatdescribed in Example 1. The polymeric compositions were in the form ofpellets having a diameter of 1.38 millimeters by 2.8 millimeters longand were obtained by cutting the strands of the cooled material intosegments as it was formed. In each of a series of experiments a portionof the granular polymeric composition was immersed in water at atemperature as stated in the following table for a period of 20 minutes,then removed and cooled to room temperature. The so-treated granuleswere measured to determine the average diameter and length of the same.Thereafter, a portion of the treated granules was foamed by heating attemperatures between 195 and 109 C. in superheated steam at atmosphericpressure for a period of three minutes. The foamed particles wereexamined to determine the ratio of the average volume of the foamedpolymer to the initial volume of the heat-treated polymeric material andthe size of the cells in the foamed polymer. Table III identifies theexperiments and gives the temperature and time for which theheat-treatment was carried out. The table also gives the averagediameter and length of the heat-treated polymeric product, the ratio ofthe volume of the foamed polymer to the initial volume of the polymericmaterial prior to foaming and the size of the cells in the foamedproduct.

Table III Heat Treatment Particle Size Foamed Product Run Foamed Den-No. Water Time, Dia., Length, Volume] Size of sity, Temp., Min. mm. mm.initial cells, lbs./

0. volume mm. on. it.

Ratio 1. 38 2. 8O 39 2. 84 1. 6 50 20 1. 14 2.07 43 0. 24 1. 45 6O 20 1.33 1. 71 43 1. 45 65 20 1. 54 l. 55 40 0.2 1. 56 70 20 1. 42 1. 69 46 0.078 1. 35 75 20 1. l3 1. 60 45 0.050 1. 38

This application is a continuation of our pending application Serial No.699,526, filed November 29, 1957, now abandoned.

We claim:

1. In a process for making a latent-foaming polymeric compositioncomprising a normally solid thermoplastic vinyl aromatic resin having avolatile organic compound having a molecular weight of at least 58 and aboiling point not higher than C. at atmospheric pressure, in which thepolymer is insoluble, uniformly distributed throughout, wherein auniform homogeneously heatplastified mass comprising the vinyl aromaticresin and the volatile organic compound under superatmospheric pressureis extruded into a zone of lower pressure and almost immediatelycontacted with a liquid cooling medium and cooled to a temperature below50 C., whereby the cooled extruded material is a rigid non-foamedmaterial and contains the solid polymer in oriented condition, the stepswhich consist in contacting the solid polymer containing the volatileorganic compound dispersed throughout with an aqueous liquid heattransfer medium at temperatures between 50 C. and the softening point ofthe polymer for a period of time not greater than minutes sufiicient toanneal the oriented polymer and insuflicient to cause appreciableswelling of the polymer by vapors of the volatile organic compoundcontained therein and without substantial lowering of its density, thencooling the sotreated polymeric material to a temperature below saidtreating temperature.

2. A process according to claim 2 wherein the vinyl aromatic resin is astyrene polymer.

3. In a process for making a latent-foaming polymeric compositioncomprising a normally solid thermoplastic vinyl aromatic resin having avolatile organic compound having a molecular weight of at least 58 and aboiling point not higher than 95 C. at atmospheric pressure, in whichthe polymer is insoluble, uniformly dispersed throughout, wherein auniform homogeneously heatplastified mass comprising the vinyl aromaticresin and the volatile organic compound under superatmospheric pressureis extruded into a zone of lower pressure and almost immediatelycontacted with a liquid cooling medium and cooled to a temperature below50 C. whereby the cooled extruded material has a density between 0.8 and1.8 and contains the solid polymer in oriented condition, the stepswhich consist in contacting the solid polymer containing the volatileorganic compound dispersed throughout with an aqueous liquid heattransfer medium at temperatures between 50 and 75 C. for a period oftime not greater than 180 minutes sufiicient to anneal the orientedpolymer and insuficient to cause appreciable swelling of the polymer byvapors of the volatile organic compound contained therein and withoutsubstantial lowering of its density, then cooling the so-treatedpolymeric material to a temperature below 40 C.

4. A process which comprises contacting a latent-foaming polymericcomposition consisting essentially of a solid thermoplastic vinylaromatic resin having a volatile organic compound having a molecularweight of at least 58 and a boiling point not higher than 95 C. atatmospheric pressure, in which the polymer is insoluble uniaozaem formlydispersed throughout, which polymeric composition has a density between0.8 and 1.2 and contains the polymer in an oriented condition, with anaqueous liquid heat transfer medium at temperatures between 50 and 75 C.for a period of time not greater than 180 minutes sufiicient to annealthe oriented polymer and insufficient to cause appreciable swelling ofthe polymer by vapors of the volatile organic compound contained thereinand without substantial lowering of its density, then cooling theso-treated polymeric material to a temperature below 40 C.

5. A process as claimed in claim 2, wherein the vinyl aromatic resin ispolystyrene and the volatile organic compound is a saturated aliphatichydrocarbon containing from 4 to 7 carbon atoms in the molecule.

6. A process as claimed in claim 3, wherein the saturated aliphatichydrocarbon is one containing 5 carbon atoms in the molecule.

References Cited in the file of this patent UNITED STATES PATENTS2,787,809 Stastny Apr. 9, 1957

1. IN A PROCESS FOR MAKING A LATENT-FOAMING POLYMERIC COMPOSITIONCOMPRISING A NORMALLY SOLID THERMOPLASTIC VINYL AROMATIC RESIN HAVING AVOLATILE ORGANIC COMPOUND HAVING A MOLECULAR WEIGHT OF A LEAST 58 AND ABOILING POINT NOT HIGHER THAN 95*C. AT ATMOSPERHIC PRESSURE, IN WHICHTHE POLYMER IS INSOLUBLE, UNLIFORMLY DISTRIBUTED THROUGHOUT, WHEREIN AUNIFORM HOMOGENEOUSLY HEATPLASTIFIED MASS COMPRISING THE VINYL AROMATICRESIN AND THE VOLATILE ORGANIC COMPOUND UNDER SUPERATMOSPHERIC PRESSUREIS EXTRUDED INTO A ZONE OF LOWER PRESSURE AND ALMOST IMMEDIATELYCONTACTED WITH A LIQUID COOLING MEDIUM AND COOLED TO A TEMPERATURE BELOW50*C., WHEREBY THE COOLED EXTRUDED MATERIAL IS RIGIN NON-FOAMED MATERIALAND CONTAINS THE SOLID POLYMR IN ORIENTED CONDITION, THE STEPS WHICHCONSIST IN CONTACTING THE SOLID POYLMER CONTAINING THE VOLATILE ORGANICCOMPOUND DISPERSED THROUGHOUT WITH AN AQUEOUS LIQUID HEAT TRANSFERMEDIUM AT TERMPERATURES BETWEEN 50*C. AND THE SOFTENING POINT OF THEPOLYMER FOR A PERIOD OF TIME NOT GREATER THAN 180 MINUTES SUFFICIENT TOANNEAL THE ORIENTED POLYMER AND INSUFFICIENT TO CAUSE APPRECIABLESWELLING OF TEH JPOLYMER BY VAPORS OF THE VOLATILE ORGANIC COMPOUNDCONTAINED THEREIN AND WITHOUT SUBSTANTIAL LOWERING OF ITS DENSITY, THENCOOLING THE SOTREATED POLYMER MATERIAL TO A TEMPERATURE BELOW SAIDTREATING TEMPERATURE.